Gas cell, quantum interference device, atomic oscillator, electronic device, and moving object

ABSTRACT

A gas cell includes an internal space in which metal atoms and a buffer gas are sealed. The buffer gas includes a gas mixture including nitrogen gas and argon gas. The mole fraction of the argon gas in the gas mixture is equal to or greater than 15% and equal to or less than 40%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/576,795, filed on Dec. 19, 2014, which claims priority to JapanesePatent Application No. 2013-263488, filed on Dec. 20, 2013, both ofwhich are hereby expressly incorporated by reference herein in theirentireties.

BACKGROUND

1. Technical Field

The present invention relates to a gas cell, a quantum interferencedevice, an atomic oscillator, an electronic device, and a moving object.

2. Related Art

An atomic oscillator which oscillates on the basis of energy transitionof atoms of an alkali metal such as rubidium or cesium is known as anoscillator having excellent long-term frequency stability.

In general, an operation principle of the atomic oscillator is mainlyclassified into a method that uses a double resonance phenomenon oflight and microwaves and a method that uses a quantum interferenceeffect (coherent population trapping (CPT)) of two types of light havingdifferent frequencies.

In a typical atomic oscillator, the alkali metal is sealed in a gas cellalong with a buffer gas (for example, see JP-A-2010-245805). Forexample, in JP-A-2010-245805, in a case of using a gas mixture ofnitrogen and argon as the buffer gas, the relationship between themixing ratio and the temperature coefficient (temperaturecharacteristics) of the gas mixture is described.

Recently, due to the demand for a reduction in the size of the atomicoscillator, a reduction in the size of the gas cell is desired.

However, in a case where the gas cell is reduced in size, even when themixing ratio of the gas mixture during the use of the gas mixture ofnitrogen and argon as the buffer gas is adjusted by using therelationship described in JP-A-2010-245805, there is a problem thattemperature characteristics cannot be improved. It is inferred that thisis because the relationship between the mixing ratio and the temperaturecharacteristics is changed according to the size of the gas cell.

SUMMARY

An advantage of some aspects of the invention is that it provides a gascell, a quantum interference device, and an atomic oscillator capable ofrealizing excellent temperature characteristics even in a case of areduction in size, and provides an electronic device and a moving objecthaving the gas cell and excellent reliability.

The invention can be implemented as the following forms or applicationexamples.

Application Example 1

This application example is directed to a gas cell including: aninternal space in which metal atoms and a buffer gas are sealed, inwhich the buffer gas is a gas mixture including nitrogen gas and argongas, and a mole fraction of the argon gas in the gas mixture is in arange equal to or greater than 15% and equal to or less than 40%.

According to the gas cell, in a case of a reduction in size, excellenttemperature characteristics (temperature coefficient) of ±0.3 Hz/°C.·Torr or less can be realized (a change in frequency with temperatureis reduced).

Application Example 2

In the gas cell according to the application example, it is preferablethat the mole fraction of the argon gas in the gas mixture is in a rangeequal to or greater than 20% and equal to or less than 37%.

With this configuration, excellent temperature characteristics of ±0.2Hz/° C.·Torr or less can be realized.

Application Example 3

In the gas cell according to the application example, it is preferablethat the metal atoms are cesium atoms.

With this configuration, the gas cell which can be used in an atomicoscillator in a method that uses a double resonance phenomenon or amethod that uses a quantum interference effect can be relatively easilyrealized.

Application Example 4

It is preferable that the gas cell according to the application examplefurther includes: a pair of window portions; and a body portion which isdisposed between the pair of window portions and forms the internalspace along with the pair of window portions, in which a distancebetween the pair of window portions is equal to or less than 10 mm.

With this configuration, the small gas cell can be provided. Inaddition, in the small gas cell, the partial pressure of the buffer gasis increased in order to exhibit excellent characteristics, and thus thetemperature characteristics of the buffer gas are significantlydifferent from those of a large gas cell. Therefore, by applying theinvention to the small gas cell, the above-described effects becomesignificant.

Application Example 5

In the gas cell according to the application example, it is preferablethat a width of the body portion along a direction perpendicular to adirection in which the pair of window portions are arranged is equal toor less than 10 mm.

With this configuration, the small gas cell can be provided. Inaddition, in the small gas cell, the partial pressure of the buffer gasis increased in order to exhibit excellent characteristics, and thus thetemperature characteristics of the buffer gas are significantlydifferent from those of a large gas cell. Therefore, by applying theteachings herein to the small gas cell, the above-described effectsbecome significant.

Application Example 6

This application example is directed to a quantum interference deviceincluding: the gas cell according to the application example.

With this configuration, even in a case of a reduction in size,excellent temperature characteristics can be realized.

Application Example 7

This application example is directed to an atomic oscillator including:the gas cell according to the application example.

With this configuration, even in a case of a reduction in size,excellent temperature characteristics can be realized.

Application Example 8

This application example is directed to an electronic device including:the gas cell according to the application example.

With this configuration, the electronic device having excellentreliability can be provided.

Application Example 9

This application example is directed to a moving object including: thegas cell according to the application example.

With this configuration, the moving object having excellent reliabilitycan be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described with reference to theaccompanying drawings, wherein like numbers reference like elements.

FIG. 1 is a schematic view illustrating an atomic oscillator (quantuminterference device) according to an embodiment of the invention.

FIG. 2 is a view illustrating an energy state of an alkali metal.

FIG. 3 is a graph illustrating the relationship between a difference infrequency between two lights emitted by a light emitting unit and theintensity of light detected by a light detecting unit.

FIG. 4 is a longitudinal cross-sectional view of a gas cell provided inthe atomic oscillator illustrated in FIG. 1.

FIG. 5 is a transverse cross-sectional view of the gas cell illustratedin FIG. 4.

FIG. 6 is a graph showing the relationship between the mole fraction andthe temperature coefficient of argon in a buffer gas including nitrogenand argon.

FIG. 7 is a view illustrating the schematic configuration of a casewhere the atomic oscillator is used in a positioning system that uses aGPS satellite.

FIG. 8 is a view illustrating an example of a moving object according.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a gas cell, a quantum interference device, an atomicoscillator, an electronic device, and a moving object according to theinvention will be described in detail on the basis of an embodimentillustrated in the accompanying drawings.

1. Atomic Oscillator (Quantum Interference Device)

First, the atomic oscillator (the atomic oscillator including thequantum interference device) will be described. In addition,hereinafter, an example in which the quantum interference device isapplied to the atomic oscillator will be described. However, the quantuminterference device according to the invention is not limited thereto,and for example, may also be applied to a magnetic sensor, a quantummemory, and the like in addition to the atomic oscillator.

FIG. 1 is a schematic view illustrating the atomic oscillator (quantuminterference device) according to the embodiment of the invention. FIG.2 is a view illustrating an energy state of an alkali metal. FIG. 3 is agraph illustrating the relationship between a difference in frequencybetween two lights emitted by a light emitting unit and the intensity oflight detected by a light detecting unit.

An atomic oscillator 1 illustrated in FIG. 1 is an atomic oscillatorthat uses a quantum interference effect.

As illustrated in FIG. 1, the atomic oscillator 1 includes a gas cell 2(gas cell), a light emitting unit 3, optical components 41, 42, 43, and44, a light detecting unit 5, a heater 6, a temperature sensor 7, amagnetic field generating unit 8, and a controller 10.

First, the principle of the atomic oscillator 1 is simply described.

As illustrated in FIG. 1, in the atomic oscillator 1, the light emittingunit 3 emits an excitation light LL toward the gas cell 2, and theexcitation light LL transmitted by the gas cell 2 is detected by thelight detecting unit 5.

An alkali metal (metal atoms) in a gas phase is sealed in the gas cell2, and as illustrated in FIG. 2, the alkali metal has three-level systemenergy levels and thus can be in three states, which are two groundstates (ground states 1 and 2) having different energy levels and anexcited state. Here, the ground state 1 is an energy state lower thanthe ground state 2.

The excitation light LL emitted by the light emitting unit 3 includestwo types of resonance lights 1 and 2 having different frequencies. Whenthe two types of resonance lights 1 and 2 irradiate the alkali metal inthe gas phase as described above, the light absorptance (lighttransmittance) of the resonance lights 1 and 2 through the alkali metalis changed according to the difference (ω₁−ω₂) between a frequency ω₁ ofthe resonance light 1 and a frequency ω₂ of the resonance light 2.

In addition, when the difference (ω₁−ω₂) between the frequency ω₁ of theresonance light 1 and the frequency ω₂ of the resonance light 2 matchesa frequency corresponding the energy difference between the ground state1 and the ground state 2, excitations from the ground state 1 and theground state 2 to the excited state are stopped. At this time, neitherof the resonance lights 1 and 2 is absorbed by the alkali metal and passtherethrough. This phenomenon is called a CPT phenomenon or anelectromagnetically induced transparency (EIT) phenomenon.

For example, when the light emitting unit 3 fixes the frequency ω₁ ofthe resonance light 1 and changes the frequency ω₂ of the resonancelight 2, in a case where the difference (ω₁−ω₂) between the frequency ω₁of the resonance light 1 and the frequency ω₂ of the resonance light 2matches a frequency ω₀ corresponding to the energy difference betweenthe ground state 1 and the ground state 2, the detection intensity ofthe light detecting unit 5 is steeply increased as illustrated in FIG.3. This steep signal is detected as an EIT signal. The EIT signal has aninherent value determined by the type of alkali metal. Therefore, theoscillator can be configured by using the EIT signal.

Hereinafter, each unit of the atomic oscillator 1 will be sequentiallydescribed.

Gas Cell

The alkali metal in the gas phase is sealed in the gas cell 2. Inaddition, a buffer gas is sealed in the gas cell 2 along with the alkalimetal gas.

Particularly, the gas cell 2 is small, and a gas mixture includingnitrogen gas and argon gas in a predetermined mixing ratio is used asthe buffer gas. Accordingly, the atomic oscillator 1 realizes excellenttemperature characteristics. The details of the gas cell 2 will bedescribed later.

Light Emitting Unit

The light emitting unit 3 (light source) has a function of emitting theexcitation light LL which excites alkali metal atoms in the gas cell 2.

More specifically, the light emitting unit 3 emits the two types oflights (the resonance light 1 and resonance light 2) having differentfrequencies described above as the excitation light LL.

The resonance light 1 can excite (resonate) the alkali metal in the gascell 2 from the above-mentioned ground state 1 to the excited state. Theresonance light 2 can excite (resonate) the alkali metal in the gas cell2 from the ground state 2 to the excited state.

The light emitting unit 3 is not particularly limited as long as it canemit excitation light as described above, and for example, asemiconductor laser such as a vertical-cavity surface-emitting laser(VCSEL) or the like may be used.

The light emitting unit 3 is adjusted to have a predeterminedtemperature (for example, about 40° C.) by a temperature adjustmentelement (heating resistor, Peltier element, and the like) notillustrated.

Optical Components

The plurality of optical components 41, 42, 43, and 44 are provided on alight path of the excitation light LL between the light emitting unit 3and the gas cell 2 described above.

Here, the optical component 41, the optical component 42, the opticalcomponent 43, and the optical component 44 are provided in this orderfrom the light emitting unit 3 to the gas cell 2.

The optical component 41 is a lens. Accordingly, the excitation light LLcan irradiate the gas cell 2 without waste.

In addition, the optical component 41 has a function of transforming theexcitation light LL into a parallel light. Accordingly, the excitationlight LL can be simply and reliably prevented from being reflected fromthe inner wall of the gas cell 2. Therefore, the resonance of theexcitation light in the gas cell 2 is appropriately generated, and as aresult, oscillation characteristics of the atomic oscillator 1 can beenhanced.

The optical component 42 is a polarizing plate. Accordingly,polarization of the excitation light LL from the light emitting unit 3can be adjusted to a predetermined direction.

The optical component 43 is a neutral density filter (ND filter).Accordingly, the intensity of the excitation light LL that is incidenton the gas cell 2 can be adjusted (reduced). Therefore, even in a casewhere the output of the light emitting unit 3 is high, the excitationlight that is incident on the gas cell 2 can be set to a predeterminedlight amount. In this embodiment, the intensity of the excitation lightLL which passes through the above-described optical component 42 and haspolarization in the predetermined direction is adjusted by the opticalcomponent 43.

The optical component 44 is a quarter-wave plate. Accordingly, theexcitation light LL from the light emitting unit 3 can be converted fromlinear polarization into circular polarization (right-handed circularpolarization or left-handed circular polarization).

In a state where Zeeman splitting occurs in the alkali metal atoms inthe gas cell 2 in the presence of a magnetic field of the magnetic fieldgenerating unit 8, as described later, when the linearly polarizedexcitation light irradiates the alkali metal atoms, due to theinteraction between the excitation light and the alkali metal atoms, thealkali metal atoms are uniformly dispersed in a plurality of levels inwhich Zeeman splitting occurs. As a result, the number of alkali metalatoms in a desired energy level is relatively smaller than the number ofalkali metal atoms in the other levels. Accordingly, the number of atomsthat exhibit a desired EIT phenomenon is reduced, and the strength ofthe desired EIT signal is reduced, resulting in the degradation of theoscillation characteristics of the atomic oscillator 1.

In the state where Zeeman splitting occurs in the alkali metal atoms inthe gas cell 2 in the presence of the magnetic field of the magneticfield generating unit 8, as described later, when the circularlypolarized excitation light irradiates the alkali metal atoms, due to theinteraction between the excitation light and the alkali metal atoms, thenumber of alkali metal atoms in a desired energy level among theplurality of levels in which Zeeman splitting occurs can be relativelylarger than the number of alkali metal atoms in the other energy levels.Therefore, the number of atoms that exhibit the desired EIT phenomenonis increased, and the strength of the desired EIT signal is increased,resulting in the improvement in the oscillation characteristics of theatomic oscillator 1.

Light Detecting Unit

The light detecting unit 5 has a function of detecting the intensity ofthe excitation light LL (the resonance light 1 and resonance light 2)that passes through the inside of the gas cell 2.

The light detecting unit 5 is not particularly limited as long as it candetect the excitation light as described above, and for example, a lightdetector (light receiving element) such as a solar cell or a photodiodemay be used.

Heater

The heater 6 (heating unit) has a function of heating theabove-described gas cell 2 (more specifically, the alkali metal in thegas cell 2). Accordingly, the alkali metal in the gas cell 2 can bemaintained in a gas phase at an appropriate concentration.

The heater 6 generates heat by conduction (direct current), and forexample, includes a heating resistor.

In addition, the heater 6 need not come into contact with the gas cell 2as long as it can heat the gas cell 2, and in this case, is provided tocome into contact with the gas cell 2 via a member (for example, amember made of metal) having excellent thermal conductivity. Instead ofthe heater 6 or in addition to the heater 6, a Peltier element may beused to heat the gas cell 2.

Temperature Sensor

The temperature sensor 7 detects the temperature of the heater 6 or thegas cell 2. On the basis of the detection result of the temperaturesensor 7, a heating value of the above-described heater 6 is controlled.Accordingly, the alkali metal atoms in the gas cell 2 can be maintainedat a desired temperature.

The installation position of the temperature sensor 7 is notparticularly limited and, for example, may be on the heater 6, or may beon the outer surface of the gas cell 2.

The temperature sensor 7 is not particularly limited and various knowntemperature sensors such as a thermistor or a thermocouple may be used.

Magnetic Field Generating Unit

The magnetic field generating unit 8 has a function of generating amagnetic field which causes Zeeman splitting to occur in the pluralityof degenerate energy levels of the alkali metal atoms in the gas cell 2.Accordingly, gaps between the different energy levels of the alkalimetal atoms which are degenerate are widened by the Zeeman splitting,and thus resolution can be enhanced. As a result, the accuracy of theoscillation frequency of the atomic oscillator 1 can be increased.

The magnetic field generating unit 8 is configured by, for example, aHelmholtz coil which is disposed to interpose the gas cell 2 therein, oris configured by a solenoid coil which is disposed to cover the gas cell2. Accordingly, a uniform magnetic field in a single direction can begenerated in the gas cell 2.

The magnetic field generated by the magnetic field generating unit 8 isa constant magnetic field (direct current magnetic field), and analternating current magnetic field may also be superposed thereon.

Controller

The controller 10 has a function of controlling the light emitting unit3, the heater 6, and the magnetic field generating unit 8.

The controller 10 includes an excitation light controller 12 whichcontrols the frequencies of the resonance lights 1 and 2 of the lightemitting unit 3, a temperature controller 11 which controls thetemperature of the alkali metal in the gas cell 2, and a magnetic fieldcontroller 13 which controls the magnetic field from the magnetic fieldgenerating unit 8.

The excitation light controller 12 controls the frequencies of theresonance lights 1 and 2 emitted by the light emitting unit 3 on thebasis of the detection result of the light detecting unit 5 describedabove. More specifically, the excitation light controller 12 controlsthe frequencies of the resonance lights 1 and 2 emitted by the lightemitting unit 3 so that the (ω₁−ω₂) described above becomes a naturalfrequency ω₀ of the alkali metal described above.

In addition, although not illustrated in the figure, the excitationlight controller 12 includes a voltage controlled crystal oscillator(oscillation circuit), and outputs an oscillation frequency of thevoltage controlled crystal oscillator as an output signal of the atomicoscillator 1 while performing synchronization and adjustment thereon onthe basis of the detection result of the light detecting unit 5.

In addition, the temperature controller 11 controls conduction to theheater 6 on the basis of the detection result of the temperature sensor7. Accordingly, the gas cell 2 can be maintained in a desiredtemperature range (for example, about 70° C.)

The magnetic field controller 13 controls conduction to the magneticfield generating unit 8 so that the magnetic field generated by themagnetic field generating unit 8 becomes constant.

The controller 10 is, for example, provided in an IC chip mounted on asubstrate.

The atomic oscillator 1 configured as described above employs a methodwhich uses the quantum interference effect, and does not need a cavityfor resonating microwaves as in the atomic oscillator employing a methodwhich uses a double resonance phenomenon. Therefore, the atomicoscillator 1 can easily achieve a reduction in size compared to theatomic oscillator which uses the double resonance phenomenon.

In order to increase short-term frequency stability, the above-describedEIT signal has a small line width (half width) and high strength.

However, when the gas cell 2 is reduced in size to reduce the size ofthe entire atomic oscillator 1, the number of alkali metal atoms whichcontribute to the above-described EIT phenomenon is reduced. Therefore,the strength of the EIT signal is reduced, and a signal-to-noise ratiois deteriorated. In addition, when the strength of the EIT signal isincreased, the effect of the interaction between the alkali metal atomsis increased, and the line width of the EIT signal is increased. Forthis reason, when the gas cell 2 is simply reduced in size, necessaryshort-term frequency stability cannot be secured.

Here, in the atomic oscillator 1, the high-pressure buffer gas is sealedin the gas cell 2. When the pressure of the buffer gas is increased, theinteraction between the alkali metal atoms can be effectively suppressedby the buffer gas. Therefore, desired short-term frequency stability canbe secured.

In addition, in the atomic oscillator 1, by using the gas mixtureincluding the nitrogen gas and the argon gas as the buffer gas,temperature characteristics are improved. Hereinafter, the gas cell 2will be described in detail.

Detailed Description of Gas Cell

FIG. 4 is a longitudinal cross-sectional view of the gas cell providedin the atomic oscillator illustrated in FIG. 1, and FIG. 5 is atransverse cross-sectional view of the gas cell illustrated in FIG. 4.FIG. 6 is a graph showing the relationship between the mole fraction andthe temperature coefficient of the argon in the buffer gas including thenitrogen and the argon.

In FIG. 4 and FIG. 5, for convenience of description, the X axis, the Yaxis, and the Z axis are illustrated as three axes which areperpendicular to each other, the tip end side of each illustrated arrowis referred to as “+(positive)”, and the base end side thereof isreferred to as “−(negative)”. Hereinafter, for convenience ofdescription, a direction parallel to the X axis is referred to as an“X-axis direction”, a direction parallel to the Y axis is referred to asa “Y-axis direction”, and a direction parallel to the Z axis is referredto as a “Z-axis direction”.

First, a portion of the gas cell 2 which is a container in which thealkali metal and the buffer gas are sealed will be described.

As illustrated in FIGS. 4 and 5, the gas cell 2 includes a body portion21 and a pair of window portions 22 and 23 which are provided with thebody portion 21 interposed therebetween.

In the body portion 21, a columnar through-hole 211 that penetratesthrough the body portion 21 in the Z-axis direction, a recessed portion212 which is open to the +Z axis direction side, and a groove 213 whichis open to the −Z axis direction side and allows the through-hole 211and the recessed portion 212 to communicate with each other are formed.

The window portion 22 is bonded to the end surface of the body portion21 on the −Z axis direction side, and the window portion 23 is bonded tothe end surface of the body portion 21 on the +Z axis direction side.Accordingly, a space S1 is formed by the through-hole 211, a space S2 isformed by the recessed portion 212, and a space S3 is formed by thegroove 213. That is, the gas cell 2 includes the spaces S1, S2, and S3which communicate with each other, and the spaces S1 and S2 communicatewith each other via the space S3.

An alkali metal Gm in a gas phase and a buffer gas Gb are sealed in thespaces S1, S2, and S3. In addition, an alkali metal M in a liquid orsolid phase is accommodated in the space S2.

The alkali metal Gm in the gas phase accommodated in the space S1 isexcited by the excitation light LL.

The temperature of the space S2 is adjusted to be lower than the spaceS1. Accordingly, the extra alkali metal Gm can be stored as the alkalimetal M in the liquid or solid phase in the space S2. In addition, thealkali metal in the liquid or solid phase can be prevented from beingprecipitated (condensed) in the space S1.

Since the space S2 communicates with the space S1 via the space S3, in acase where the alkali metal Gm in the gas phase in the space S1 isreduced by reaction with the inner wall surface of the gas cell 2 or thelike, the alkali metal M in the space S2 is vaporized, and thus theconcentration of the alkali metal Gm in the gas phase in the space S1can be maintained at a constant level.

A material which forms the window portions 22 and 23 of the gas cell 2is not particularly limited as long as it has a property of transmittingthe excitation light described above, and for example, a glass material,crystal, or the like may be used. Depending on the thicknesses of thewindow portions 22 and 23 or the intensity of the excitation light, thewindow portions 22 and 23 may be made of silicon.

Likewise, a material which forms the body portion 21 of the gas cell 2is not particularly limited and similarly to the window portions 22 and23, may be a glass material, crystal, or the like, and may also be ametal material, a resin material, a silicon material, or the like. Amongthese, as a constituent material of the body portion 21, any of a glassmaterial, crystal, or a silicon material is preferably used, and asilicon material is more preferably used. Accordingly, even in a case offorming small gas cells 2 which are equal to or less than 10 mm in widthor height, highly accurate body portions 21 can be easily formed byusing a microfabrication technique such as etching. In addition, thebody portion 21 made of the silicon material can be simply andairtightly bonded to the window portions 22 and 23 made of the glassmaterial by anodic bonding.

The method of bonding the body portion 21 to the window portions 22 and23 of the gas cell 2 is determined by the constituent materials thereof,and is not particularly limited as long as the materials can beairtightly bonded to each other. For example, a bonding method using anadhesive, direct bonding, anodic bonding, and the like may be used.

As described above, in the internal space formed by the spaces S1, S2,and S3 of the gas cell 2 (hereinafter, simply referred to as an“internal space”), a gas mixture of the alkali metal Gm in the gas phaseand the buffer gas Gb is sealed. Here, the spaces S2 and S3 may beomitted so that the alkali metal M in the liquid or solid phase may beprecipitated in the space S1. In this case, the temperature distributionof the gas cell 2 may be managed so as not to precipitate the alkalimetal M in the region through which the excitation light LL passes.

Subsequently, the alkali metal and the buffer gas will be described.

In the gas cell 2, cesium is preferably used as the sealed alkali metal.By using cesium atoms as the alkali metal atoms, the gas cell 2 whichcan be used in the atomic oscillator 1 in the method that uses thedouble resonance phenomenon or the method that uses the quantuminterference effect can be relatively easily realized.

In the gas cell 2, the gas mixture including the nitrogen gas and theargon gas is preferably used as the sealed buffer gas. In addition, inthe gas mixture forming the buffer gas, it is preferable that a gasother than the nitrogen gas and the argon gas be not included. However,for example, a very small amount of helium, neon, krypton, or the likeat a degree that does not affect the frequency and temperaturecharacteristics may be included.

Here, in a case of using cesium atoms as the alkali metal atoms, whenhelium, neon, or nitrogen is singly used as the buffer gas, positivetemperature characteristics in which frequency increases as temperatureincreases are exhibited in a case where the other temperaturecharacteristics of the atomic oscillator 1 are ignored. In a case ofsingly using argon or krypton as the buffer gas, negative temperaturecharacteristics in which frequency decreases as temperature increasesare exhibited in a case where the other temperature characteristics ofthe atomic oscillator 1 are ignored.

Therefore, by adjusting the mixing ratio of the nitrogen gas and theargon gas in the buffer gas, temperature characteristics can beimproved. As a method of improving the temperature characteristics, theuse of a well-known mixing ratio has been suggested.

However, in a case where the gas cell 2 is reduced in size, thetemperature characteristics cannot be improved even when using thewell-known mixing ratio.

The inventors have intensively conducted examinations, and as a result,obtained the knowledge that the relationship between the mixing ratioand the temperature characteristics is changed when the gas cell 2 isreduced in size to a predetermined degree or less. Specifically, therelationship between the mixing ratio and the temperature coefficient asshown in FIG. 6 was found.

Here, the buffer gas used in the gas cell 2 is the gas mixture includingthe nitrogen gas and the argon gas in which the mole fraction of theargon gas is equal to or greater than 15% and equal to or less than 40%.Accordingly, in a case where the gas cell 2 is reduced in size,excellent temperature characteristics of ±0.3 Hz/° C.·Torr or less canbe realized (a change in frequency with temperature is reduced). Here,“the mole fraction of the argon gas” is a ratio of the number of moles(partial pressure) of the argon gas to the number of all moles (totalpressure) of the gas forming the buffer gas. In addition, the gasmixture of the buffer gas and the alkali metal is sealed in the gas cell2. However, the partial pressure of the alkali metal gas is equal to thevapor pressure of the alkali metal, and is thus extremely smaller thanthe partial pressure of the buffer gas. Accordingly, the partialpressure of the buffer gas can approximate the pressure of the gasmixture of the buffer gas and the alkali metal gas, that is, thepressure in the gas cell. Therefore, in a case of using a high-pressurebuffer gas, the partial pressure of the alkali metal gas can be ignored.

Furthermore, the mole fraction of the argon gas in the gas mixtureforming the buffer gas is preferably equal to or greater than 20% andequal to or less than 37% or less, and more preferably equal to orgreater than 25% and equal to or less than 30%. In a case where the molefraction is equal to or greater than 20% and equal to or less than 37%,excellent temperature characteristics of ±0.2 Hz/° C.·Torr or less canbe realized. In addition, in a case where the mole fraction is equal toor greater than 25% and equal to or less than 30%, excellent temperaturecharacteristics of ±0.1 Hz/° C.·Torr or less can be realized.

The partial pressure of the buffer gas is preferably equal to or greaterthan 30 Torr and equal to or less than 100 Torr, and is more preferablyequal to or greater than 30 Torr and equal to or less than 80 Torr.Accordingly, the action of the buffer gas which suppresses theinteraction between the alkali metal atoms as described above can beeffectively exhibited. Here, when the partial pressure of the nitrogengas or the buffer gas is too low, the line width of the EIT signal has atendency to increase. Contrary to this, when the partial pressure of thenitrogen gas or the buffer gas is too high, the intensity of the EITsignal has a tendency to decrease. In addition, the gas cell 2 isstrengthened, and thus it is difficult to achieve a reduction in size.

From the result shown in FIG. 6, it is preferable that the buffer gasdoes not substantially include neon gas. Accordingly, the partialpressure of the buffer gas can be stabilized over a long period of time.Here, “neon gas is not substantially included” includes both a casewhere the buffer gas absolutely does not include the neon gas and a casewhere the buffer gas includes a very small amount of neon gas at adegree that does not affect the frequency stability.

In the gas cell 2 in which the buffer gas is sealed as described above,the length of the internal space in the direction (Z axis direction)between the pair of window portions 22 and 23 is equal to or less than10 mm. In addition, the width of the internal space of the gas cell 2along the direction (x axis direction) parallel to the pair of windowportions 22 and 23 is equal to or less than 10 mm. Accordingly, thesmall gas cell 2 can be provided. In addition, in the small gas cell 2,the partial pressure of the buffer gas is increased in order to exhibitexcellent characteristics, and thus the temperature characteristics ofthe buffer gas are significantly different from those of a large gascell. Therefore, by applying the teachings herein to the small gas cell2, the above-described effects become significant.

2. Electronic Device

The atomic oscillator described above may be assembled into varioustypes of electronic devices. The electronic devices have excellentreliability.

Hereinafter, an exemplary electronic device will be described.

FIG. 7 is a view illustrating the schematic configuration of a casewhere the atomic oscillator is used in a positioning system that uses aGPS satellite.

A positioning system 100 illustrated in FIG. 7 includes a GPS satellite200, a base station device 300, and a GPS reception device 400.

The GPS satellite 200 transmits positioning information (GPS signal).

The base station device 300 includes a reception device 302 whichreceives the positioning information from the GPS satellite 200 at highaccuracy via an antenna 301 provided in, for example, an electronicreference point (GPS connection observation station), and a transmissiondevice 304 which transmits the positioning information received by thereception device 302 via an antenna 303.

Here, the reception device 302 is an electronic device provided with theabove-described atomic oscillator 1 as a reference frequency oscillationsource. The reception device 302 has excellent reliability. In addition,the positioning information received by the reception device 302 istransmitted by the transmission device 304 in real time.

The GPS reception device 400 includes a satellite reception unit 402which receives the positioning information from the GPS satellite 200via an antenna 401, and a base station reception unit 404 which receivesthe positioning information from the base station device 300 via anantenna 403.

3. Moving Object

FIG. 8 is a view illustrating an example of the moving object.

In this figure, a moving object 1500 includes a vehicle body 1501 andfour wheels 1502, and is configured so that the wheels 1502 are rotatedby a driving source (engine) (not illustrated) provided in the vehiclebody 1501. The atomic oscillator 1 is embedded into the moving object1500.

According to the moving object, excellent reliability can be exhibited.

The electronic device which includes the atomic oscillator according tothe invention is not limited to the above description, and for example,can be applied to a cell phone, a digital still camera, an inkjet typedischarge device (for example, an ink jet printer), a personal computer(mobile personal computer or laptop personal computer), a television, avideo camera, a video tape recorder, a car navigation device, a pager,an electronic organizer (including a communication function), anelectronic dictionary, a calculator, an electronic game equipment, aword processor, a workstation, a videophone, a security televisionmonitor, electronic binoculars, a POS terminal, medical instruments (forexample, an electronic thermometer, a blood pressure gauge, a bloodsugar meter, an electrocardiogram display device, an ultrasonicdiagnostic device, an electronic endoscope, and the like), a fishfinder, various types of measuring apparatuses, measuring gauges (forexample, measuring gauges of vehicles, airplanes, and ships), a flightsimulator, terrestrial digital broadcasting, a cell phone base station,and the like.

Hereinabove, the gas cell, the quantum interference device, the atomicoscillator, the electronic device, and the moving object according tothe invention have been described on the basis of the illustratedembodiment, and the invention is not limited thereto.

In addition, the configuration of each unit of the invention can besubstituted with an arbitrary configuration which exhibits the samefunction as the above-described embodiment, or an arbitraryconfiguration may be added.

In addition, in the above-described embodiment, the quantum interferencedevice which causes resonance transition of cesium or the like by usingthe quantum interference effect of two types of light having differentfrequencies is described as an example of the quantum interferencedevice according to the invention. However, the quantum interferencedevice according to the invention is not limited thereto, and a doubleresonance device which causes resonance transition of rubidium or thelike by using the double resonance phenomenon of light and microwavesmay also be applied.

What is claimed is:
 1. A gas cell comprising: a body having a firstrecess, a second recess and a groove, the first recess being open to afirst end of the body, and the groove interconnecting the first recessand the second recess; a window bonded to the first end of the body; aninternal space being configured by the first recess, the second recess,the groove and the window; metal atoms which are sealed in the internalspace; and a buffer gas which is sealed in the internal space, whereinthe buffer gas is a gas mixture including nitrogen gas and argon gas,and a mole fraction of the argon gas in the gas mixture is in a rangeequal to or greater than 15% and equal to or less than 40%.
 2. The gascell according to claim 1, wherein the mole fraction of the argon gas inthe gas mixture is in a range equal to or greater than 20% and equal toor less than 37%.
 3. The gas cell according to claim 1, wherein themetal atoms are cesium atoms.
 4. A quantum interference devicecomprising: the gas cell according to claim
 1. 5. An atomic oscillatorcomprising: the gas cell according to claim
 1. 6. An electronic devicecomprising: the gas cell according to claim
 1. 7. A moving objectcomprising: the gas cell according to claim
 1. 8. A gas cell comprising:a body having a first recess, a second recess and a groove, the firstrecess being open to a first end of the body, and the grooveinterconnecting the first recess and the second recess; a window bondedto the first end of the body; an internal space being configured by thefirst recess, the second recess, the groove and the window; metal atomswhich are sealed in the internal space; and a buffer gas which is sealedin the internal space, wherein a distance between the window and abottom of the first recess is equal to or less than 10 mm, and a widthof the body parallel to the window is equal to or less than 10 mm. 9.The gas cell according to claim 8, wherein the buffer gas is a gasmixture including argon gas; and wherein the mole fraction of the argongas in the gas mixture is in a range equal to or greater than 20% andequal to or less than 37%.
 10. The gas cell according to claim 9,wherein the metal atoms are cesium atoms.
 11. The gas cell according toclaim 8, wherein the metal atoms are cesium atoms.